How to get rid of "Green Water"?

This is a discussion on How to get rid of "Green Water"? within the Beginner Freshwater Aquarium forums, part of the Freshwater Fish and Aquariums category; -->
Hello there!
My friend has a 30 gallon tank with two goldfish, and she has had a horrible green water problem from virtually day ...

My friend has a 30 gallon tank with two goldfish, and she has had a horrible green water problem from virtually day one. What is the best way to go about getting rid of it? Huge water changes aren't helping.

is the tank set up near a window? does it catch any sunlight? how long does your friend leave the lights on for? how often is the water changed and how much? what are the nitrate levels? can your friend test for phosphates? test the tap water parameters as well; you said she has had the problem since the beginning.

for now, you could do a large water change and just leave the lights off. if you are able to get RO water, use that. if you think there is uneaten food on the bottom then definitely find a more suitable food.

goldfish produce a lot of waste and can easily give you high nitrate levels and that can give you green water.what type of goldfish are they and how large?

is the tank set up near a window? does it catch any sunlight? It catches a minor amount of sunlight, it's in a dark corner by a north-facing window. (Wintertime in my latitude means low amounts of sunlight)

how long does your friend leave the lights on for? 9 hours a day.

how often is the water changed and how much?She changes it 100% when she can't see her fish any more. (Every week and a half)

what are the nitrate levels? can your friend test for phosphates?I can only assume her nitrates and phosphates are high because of the goldfish's waste and the bloom.

test the tap water parameters as well; you said she has had the problem since the beginning. We have the same tap water, no ammonia or anything else, it is quite hard, has a ph of 8.0.

for now, you could do a large water change and just leave the lights off. if you are able to get RO water, use that. if you think there is uneaten food on the bottom then definitely find a more suitable food.Apparently her goldfish don't eat floating food. They're pretty, but hers aren't the brightest.

goldfish produce a lot of waste and can easily give you high nitrate levels and that can give you green water.what type of goldfish are they and how large?They are fantails, she has two and their bodies are roughly 2" long.

im no expert, but i think if she did weekly 50% water changes, made sure to not overfeed, and maybe cut back the lighting to 7 hours a day, that it would clear up. i started to have green water in one of my tanks and i just cut back the lighting times and it cleared up on its own.

Coincidentally I just did a response to the same question, so for ease I'll just copy and paste it here; it was specific to that situation but the general info is the same. Byron.

Green water is caused when green unicellular algae reproduce so rapidly the water turns green. This happens because of high light and high nutrients.

I would be interested in the exact number for nitrates; I expect it is very high, certainly not "normal" or this would not occur.

The light you have is also contributing to the problem; the "blue/white" I assume is actinic or similar, a type of light that is intended for corals and reef tanks because it simulates the colour of sunlight that penetrates to those depths. It is not a good choice for freshwater, as all algae find it favourable, and plants do not do well under it.

Having a well-planted tank can prevent green water, provided the light is balanced with the nutrients such that the plants can use both completely. But it takes more than a few plants. Without plants, there is nothing to use those nutrients except algae.

A caution on using chemicals to handle algae: don't. These will not work on green water, but even with other types of true algae, they are very dangerous to the fish and not worth the risk. There is always a reason for algae, and finding it and rectifying it is the only good course.

As for getting rid of it, obviously reduce the nutrients and the light. Even if you did reduce this with major water changes, it will only return if the cause is not rectified. What is the fish load in this tank? And what is the feeding schedule? And normally how often is a partial water change carried out, and with how much volume? Knowing this information will help us advise on the solution.

I suspect overfeeding combined with the lighting and high nitrate were sort of the "perfect storm" for green water. Changing the water more often to deal with nitrate, cutting back on feeding and swapping out the lighting for something that won't promote algae growth would be a good idea. If the tank isn't planted, low-temperature white bulbs (usually referred to as "cool white" I believe) would be better than the actinics.

Your friend should stick with the sinking pellets. Definitely a better food choice for goldies than floating pellets or flakes.